on Muscular Motion. 
21 
upon the posterior part of the eyeball ; the action of these 
muscles brings the membrane over the cornea, and the in- 
stant they cease to contract, the elasticity of the membrane 
draws it back again. 
The eyes of fishes have several peculiarities, and in many 
respects their structure differs from that which is observed 
in the quadruped and bird. 
The muscles of the eye, that correspond to the straight 
muscles in the quadruped, are four in number, they are, how- 
ever, differently placed ; they do not surround the eyeball ; 
but two of them are on that side of the orbit next to the nose 
of the fish, the other two on the opposite side ; their attach- 
ment to the eye is close to the edge of the cornea; they do 
not, however, pass round the eyeball towards the posterior part, 
as in other animals, but are connected with the bones of the 
head at some distance from the eye on each side ; so that they 
cannot at all compress the eye laterally, they can only pull it 
backwards by the combined effect of their action. 
The bottom of the orbit on which the eyeball rests, is so- 
lid, and adapted to it, there being no fat interposed between 
them as in other animals ; and where the eye is removed to a 
great distance from the skull, and that cannot be the case, there 
is a strong cartilage projecting from the skull to the bottom of 
the eye, and that end of it next to the eye is concave, and 
fitted to the portion of the eyeball directly opposite the cornea, 
just above the entrance of the optic nerve. This is considered 
as a fixed point upon which the eye moves, but it will also, 
from the situation of the muscles, allow the eye' to be forced 
back upon it, and the whole eye to be flattened. 
The shape of the eye differs considerably in different fishes, 
